vCenter Server versions 5.1 and later require the vCenter Single Sign-On and vCenter Inventory Service components. Depending on your existing vCenter Server installation, you can use the Simple Install option to upgrade to vCenter Server, including Single Sign-On and Inventory Service, all on a single host machine. This option is appropriate for small deployments.

You can use Simple Install to upgrade vCenter Server if you have a version 4.x or 5.0.x vCenter Server installation that is supported for upgrade. See vCenter Server Upgrade Summary. In this case, the Simple Install option installs Single Sign-On, upgrades Inventory Service, and upgrades vCenter Server.